This invention relates to the casting of steel strip by a single or a twin roll caster. In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontally positioned casting rolls, which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a thin cast strip product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel, from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the casting pool will generally be at a temperature in excess of 1550° C., and usually 1600° C. and greater. It is necessary to achieve very rapid cooling of the molten steel over the casting surfaces of the rolls in order to form solidified shells in the short period of exposure on the casting surfaces to the molten steel casting pool during each revolution of the casting rolls. Moreover, it is important to achieve even solidification so as to avoid distortion of the solidifying shells which come together at the nip to form the steel strip. Distortion of the shells can lead to surface defects known as “crocodile skin surface roughness.” Crocodile skin surface roughness is known to occur with high carbon levels above 0.065%, and even with carbon levels below 0.065% by weight carbon. Crocodile skin roughness, as illustrated in FIG. 1, is known to occur for other reasons. Crocodile skin roughness involves periodic rises and falls in the strip surface of 40 to 80 microns, in periods of 5 to 10 millimeters, measured by profilometer.
We have found that with carbon levels below 0.065% by weight the formation of crocodile skin surface roughness is directly related to the heat flux between the molten metal and the surface of the casting rolls, and that the formation of crocodile skin roughness can be controlled by controlling the heat flux between the molten metal and the surface of the casting rolls. FIG. 2 reports dip tests that illustrates the relationship between the heat flux and the formation of crocodile skin roughness during the formation of the metal shells on the surfaces of the casting rolls in making the thin cast strip. As shown by FIG. 2, we have also found that by controlling the energy exerted by rotating brushes peripherally in contact with the casting surfaces of each casting roll, in advance of contact of the casting surface with the molten metal, that the heat flux between the molten metal and the surface of the casting rolls, and in turn crocodile skin surface roughness on the resulting thin cast strip can be controlled.
This relationship between the heat flux from the molten metal and the surface of the casting rolls and the formation of crocodile skin surface roughness on the thin cast strip has been found to occur whether the casting roll surfaces are smooth or textured. FIG. 3 reports dip tests that illustrate how the heat flux is changed with both smooth and textured casting surfaces on the casting rolls. We have also found that the texture of the casting roll surfaces of the casting rolls change during casting. This change can cause a change in heat flux from the molten metal to the casting roll surfaces and in turn a change in formation of crocodile skin surface roughness on the thin cast strip. We have found a method of directly controlling the formation of crocodile skin surface roughness by controlling the heat flux between the molten metal and the casting roll surfaces, to avoid high fluctuations in the heat flux during the formation of the metal shells during casting and in turn control the forming of crocodile skin surface roughness in the thin cast strip produced.
The method of controlling the formation of crocodile skin surface roughness in continuous casting of thin cast strip of plain carbon steel is disclosed that comprises the steps of:
assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;
forming a casting pool of molten metal of plain carbon steel of less than 0.065% by weight carbon supported on the casting surfaces of the casting rolls above the nip;
assembling a rotating brush peripherally to contact the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal in the casting pool;
forming a desired degree of cleaning of the casting surfaces of the casting rolls with a majority of projections on the casting surfaces exposed and provide wetting contact between the casting surface and the molten metal of the casting pool by controlling the energy exerted by the rotating brushes during a casting campaign;
controlling the energy exerted by the rotating brushes against the casting surfaces of the casting rolls using the desired degree of cleaning as a reference to clean the expose a majority of projections of the casting surfaces of the casting rolls and provide wetting contact between the casting surface and the molten metal of the casting pool; and counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip.
The casting surfaces of the casting rolls may be textured with projections, and the cleaning of the casting surfaces of the casting rolls maintains a majority of extended portions of said projections exposed for contact with the molten metal of the casting pool. These exposed projections of the casting surface, however, may be about one-twentieth or one-thirtieth, or less, of the surface area of the casting surface. There is still residual material, including metal and oxides, in the “valleys,” entices and other low areas of the casting surfaces, as opposed to the raised areas of the casting surfaces. More specifically, the casting surfaces of the casting rolls may be textured with a random distribution of discrete projections as described and claimed in application Ser. No. 10/077,391, filed Feb. 15, 2002 and published Sep. 12, 2002, as US 2002-0124990, the disclosure of which is incorporated by reference.
In any event, a substantial portion of the casting surface is exposed by the cleaning of the casting surfaces so that there can be wetting of the casting surface by the molten metal when the casting surface is rotated into contact with the casting pool. Cleaning here does not mean the casting surfaces are completely clean of all contaminates. Clean here means that the parts of the casting roll surfaces that are exposed, the projections, are substantially free from matter that adulterates or contaminates wetting of the casting surfaces by the molten metal and inhibits effective heat flux from the molten metal to the casting surfaces. It is not necessary or practical for the brushes to clean all exposed projections of the casting surface. Clean means that the exposed casting surfaces are sufficiently clean that the formation of crocodile skin roughness is inhibited, if not eliminated. FIGS. 9 through 11 illustrate cleaning of the casting surface to expose a majority of the projections of the surface in accordance with this invention.
The energy exerted by the cleaning brush against the casting surface of the casting roll is determined by the pressure by the brush against the casting surface and the speed of rotation of the brush and the casting speed. This may be done, for example, by measuring the throughput and/or the differential pressure of hydraulic fluid through hydraulic motors, which power the brushes cleaning the casting surfaces of the casting rolls. This may be done manually or by automated controls, and as explained below automated controls have provided the best mode contemplated of the invention.
Alternatively, a method of controlling the formation of crocodile skin surface roughness in continuous casting of thin cast strip of plain carbon steel is disclosed that comprises the steps of:
assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;
forming a casting pool of molten metal of plain carbon steel of less than 0.065% by weight carbon supported on the casting surfaces of the casting rolls above the nip;
assembling a rotating brush using hydraulic motors peripherally to contact the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal in the casting pool;
setting a desired degree of cleaning of the casting surfaces of the casting rolls with a majority of projections on the casting surfaces exposed and provide wetting contact between the casting surface and the molten metal of the casting pool by controlling the energy exerted by the rotating brushes during a casting campaign;
monitoring the torque of the hydraulic motors to control the energy exerted by the rotating brushes against the casting surfaces of the casting rolls using the desired degree of cleaning as a reference to clean the expose a majority of projections of the casting surfaces of the casting rolls and provide wetting contact between the casting surface and the molten metal of the casting pool; and
counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip.
The torque of the hydraulic motors may be monitored by measuring the pressure differential between inlet and outlet of hydraulic fluid through the hydraulic motors. Alternatively, the torque of the hydraulic motors may be monitored by measuring the torque between the hydraulic motor and a chock or a motor mount. The energy of the rotating brush against the casting roll may also be controlled by varying the rotation speed of the brush against the casting surface of the casting roll. In any event, the monitoring of the torque of the hydraulic motors, and in turn the energy exerted by the bushes against the casting surfaces, may be controlled manually or by automated controls, but the automated controls provide the best mode of performing the invention as explained by for example below.
The casting surfaces of the casting rolls may be textured projections, and in addition may be with a random distribution of discrete projections.
In an alternative, the method of controlling the formation of crocodile skin surface roughness in continuous casting of thin-cast strip may comprise the steps of:
assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;
forming a casting pool of molten metal of plain carbon steel of less than 0.065% by weight carbon supported on the casting surfaces of the casting rolls above the nip;
assembling a rotating brush peripherally capable of contacting the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal;
forming clean bands exposing a majority of the projections of the casting surfaces of the casting rolls as reference for controlling the pressure exerted by the rotating brushes against the casting surfaces of the casting rolls;
controlling the energy of the rotating brush against the casting rolls using the clean band as a reference to clean the casting surfaces; and
counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip.
The casting surfaces, of which the clean bands are a part, are typically textured. The casting surfaces have a majority of extended portions of said projections exposed for contact with the molten metal of the casting pool. However, the exposed surfaces of the clean bands are still a minor part of the area of the casting surfaces of the casting rolls. There is still residue in the “valleys,” entices and other low areas of the clean bands (as opposed to the raised areas of the clean bands) which may be the majority of the surface area. More specifically, again, the casting surfaces of the casting rolls may be textured with a random distribution of discrete projections as described and claimed in application Ser. No. 10/077,391, filed Feb. 15, 2002 and published Sep. 12, 2002, as US 2002-0124990, the disclosure of which is incorporated by reference. In any event, again, the exposed surface is not the majority of the casting surfaces or the clean bands thereof.
However, a substantial portion of the casting surface is exposed by the cleaning of the casting surfaces so that they can be wetted of the casting surface by the molten metal when the casting surface is rotated into contact with the casting pool. Further, clean here means that the parts of the casting roll surfaces that are exposed are substantially free from matter that adulterates or contaminates wetting of the casting surfaces by the molten metal, and inhibits effective heat flux from the molten metal to the casting surfaces. However, again, it is not necessary or practical for the brushes to clean all exposed projections of the casting surface. Again, clean means that the exposed casting surfaces are sufficiently clean that the formation of crocodile skin roughness is inhibited, if not eliminated. Again, FIGS. 9 and 11 illustrate cleaning of the casting surfaces to expose a majority of projections of the surfaces in accordance with this invention.
As before, the energy exerted by the cleaning brush against the casting surface of the casting roll is determined by the pressure by the brush against the casting surface and the speed of rotation of the brush and the casting speed. This can be measured and controlled by the flow of hydraulic fluid through a hydraulic motor driving rotation of the brush and in turn the speed of rotation of the brushes, and/or by pressure differential hydraulic fluid across the hydraulic motors driving the brushes, and in turn the torques of the hydraulic motors and the pressure exerted by the brushes against the casting surfaces of the casting rolls.
A further alternative, the method of controlling the formation of crocodile skin surface roughness in continuous casting of thin-cast strip of plain carbon steel comprising the steps of:
assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;
forming a casting pool of molten metal of plain carbon steel of less than 0.065% by weight carbon supported on the casting surfaces of the casting rolls above the nip;
assembling a rotating brush peripherally to contact the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal capable of cleaning residual from the surface of the casting roll;
cleaning to expose the majority of projections of the casting surfaces of the casting rolls and initially measuring the heat flux from the molten metal to the cleaned casting surfaces;
continually measuring the heat flux from the molten metal to the casting surfaces of the casting rolls;
controlling the energy exerted by the rotating brush against the casting rolls based on the difference between said measured heat flux and the initially measured heat flux between the molten metal and the casting surfaces; and
counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip.
This alternative has the advantage that the initial heat flux measured provides the reference for the clean casting surfaces of the casting rolls cleaned, as above described to serve as the reference for cleaning throughout the casting campaign. The same effective cleaning of the casting surfaces can thus be controlled and maintained through the casting campaign. In turn, the cleaning of the casting surfaces can be monitored and controlled indirectly by controlling the energy exerted by rotating brush against the casting rolls either manually or automatically as explained in detail by example below.
The energy of the rotating brush against the casting roll may be in turn controlled based on the casting speed by varying the application pressure or the speed of rotation, or both, of an electric, pneumatic or hydraulic motor rotating the brush against the casting surface. The energy of the rotating brush can be measured by measuring the torque of the motor rotating. The heat flux between the molten metal and the casting surfaces of the casting rolls may be initially measured and continually measured, as well as the difference between the real time heat flux and the initial heat flux measured, by measuring the difference in temperature of the cooling water circulated through the casting roll between the inlet and outlet as described in U.S. Pat. Nos. 6,588,493 and 6,755,234. Still it is contemplated that the heat flux can be measured by any available method. In any event, by monitoring the heat flux and calculating the difference in heat flux from the initial heat flux measured, the energy exerted by the brush against the casting surface can be automatically controlled by a control system that receives electrical signals from the monitor corresponding to the measured heat flux, and controls the energy exerted by the brush against the casting roll based on the difference in heat flux from the initial heat flux measured.
In addition, the method of controlling the formation of crocodile skin surface roughness in continuous casting of thin-cast strip may include the additional step of:
controlling the pressure of the gas blown through ports onto the casting surfaces of the casting rolls based on the difference between said measured heat flux and an initially measured heat flux between the molten metal and the casting surfaces to assist in controlling the formation of crocodile skin surface roughness in continuous casting of thin-cast strip.
Plain carbon steel for purpose of the present invention is defined as less than 0.065% carbon, less than 10.0% silicon, less than 0.5% chromium, less than 2.0% manganese, less than 0.5% nickel, less than 0.25% molybdenum, and less than 1.0% aluminum, together with other elements such as sulfur, oxygen and phosphorus which normally occur in making carbon steel by electric arc furnace. Low carbon steel may be used in these methods having a carbon content in the range 0.001% to 0.1% by weight; a manganese content in the range 0.01% to 2.0% by weight; and a silicon content in the range 0.01% to 10.0% by weight. The steel may have an aluminum content of the order of 0.01% or less by weight. The aluminum may, for example, be as little as 0.008% or less by weight. The molten steel may be a silicon/manganese killed steel.